Alstom MiCOM P546 - 2.1.2 Relay sensitivity under heavy load conditions

P54x/EN AP/La4 Application Notes
(AP) 6-10 MiCOM P543, P544, P545 & P546
AP
Voltage (kV)
Core formation
and spacing
Conductor size
in mm
2
Charging current
A/km
400 kV Cable Flat, 145 mm 2000 28
400 kV Cable Tref., 585 mm 3000 33
Table 1. Typical cable/line charging currents (UK, 50 Hz)
If capacitive charging current is disable, the setting of Is1 must be set above 2.5 times the
steady state charging current. Where charging current is low or negligible, the
recommended factory default setting of 0.2 In should be applied.
The tripping criteria can be formulated as:
1. for |
bias| < s2,
|
diff| > k1.| bias| + s1
2. for |
bias| > s2,
|
diff| > k2.| bias| - (k2 - k1). s2 + s1
2.1.2 Relay sensitivity under heavy load conditions
The
sensitivity of the relay is governed by its settings and also the magnitude of load current
in the system. For a three-ended system, with relays X, Y and Z, the following applies:
|
diff| = |(
X
+ 
Y
+ 
Z
)|
|
bias| = 0.5 (|
X
| + |
Y
| + |
Z
|)
Assume a load current of L flowing from end X to Y and Z. Assume also a high resistance
fault of current F being singly fed from end X. For worst case analysis, we can assume also
F to be in phase with L:-

X
= 
L
+ 
F

Y
= -y
L
where 0<y<1

Z
= - (1-y) 
L
|
diff| = |
F
|
|
bias| = |
L
| + 0.5 |
F
|
Relay sensitivity when |bias| < s2:
For |bias| < s2, the relay would operate if |diff| > k1 |bias| + s1
or |

F
| > k1 (|
L
| + 0.5 |
F
|) + s1
or (1 - 0.5 k1) |

F
| > (k1 |
L
| + s1)
or |

F
| > (k1 |
L
| + s1) / (1 - 0.5 k1)
For s1 = 0.2 pu, k1 = 30% and s2 =2.0 pu, then
1. for |L| = 1.0 pu, the relay would operate if |F| > 0.59 pu
2. for |L| = 1.59 pu, the relay would operate if |F| > 0.80 pu
If |F| = 0.80 pu and || = 1.59 pu, then |bias| = 1.99 pu which reaches the limit of the low
percentage bias curve.